🌿 Soil pH in water, in KCl and in CaCl₂ — potentiometric method
In short
Routine determination of pH (2–12) with a glass electrode in a 1:5 (by volume) suspension of a sample of soil, sludge or treated biowaste in water (pH-H₂O), in 1 mol/l KCl (pH-KCl) or in 0. The test is performed according to PN-EN ISO 10390:2022-09; Range: pH 2–12. The procedure comprises 5 steps; it is used for: Routine analysis of agricultural and horticultural soils — assessment of the reaction, classification of the soil, planning of liming, Monitoring of soils (the chemistry of arable soils, industrial sites, reclamation), Assessment of sewage sludges and composts (treated biowaste) before agricultural use.
At a glance
- Standard: PN-EN ISO 10390:2022-09
- Category: Environment
- Procedure steps: 5
- Range: pH 2–12
- Sample-to-solution ratio: 1:5 by volume (≥ 5 ml of sample + 25 ml of solution; biowaste without comminution: ≥ 60 ml + 300 ml)
- Solutions: water (conductivity ≤ 0.2 mS/m at 25 °C); KCl 1 mol/l (74.5 g/l); CaCl₂ 0.01 mol/l (1.47 g CaCl₂·2H₂O/l)
Overview
The earlier edition was PN-ISO 10390:1997.
The reaction of a soil is a measure of its active acidity — of the concentration of hydrogen ions in the soil solution, expressed as pH = −log[H⁺]. In soil science practice the pH is determined in a suspension of the soil with distilled water (pH-H₂O) or with an electrolyte — a solution of KCl 1 mol/l (pH-KCl) or of CaCl₂ 0.01 mol/l (pH-CaCl₂); a measurement in an electrolyte displaces the H⁺ ions from the sorption complex and gives values lower than those in water. The reaction affects the availability of nutrients, the biological activity and the physical properties of the soil, and in Poland it forms the basis of the classification of soils (very acid, acid, slightly acid, neutral, alkaline) and of the establishment of the need for liming.
ISO 10390:2021 (the third edition) replaced ISO 10390:2005; it merged the content of ISO 10390:2005 and of EN 15933:2012, extended the scope to sludge and treated biowaste and added validation data. The standard specifies an instrumental method for the routine determination of pH over the range 2–12 with a glass electrode in a 1:5 suspension (volume fraction) and is applied to all kinds of air-dry samples of soil and of treated biowaste (e.g. prepared in accordance with ISO 11464 or EN 16179); liquid sludges are measured directly. The ratio by volume was chosen so that all samples — including those of low density — should be treated alike; measuring out the sample with a spoon of known capacity is sufficiently accurate.
Method principle
A test portion (at least 5 ml, measured out with a spoon) is suspended in five times its volume of water, of a solution of KCl (c = 1 mol/l) or of CaCl₂ (c = 0.01 mol/l), shaken for 60 ± 10 min and left to stand for at least 1 h, but for no longer than 3 h, with access of air avoided. The pH of the suspension is measured with a pH meter with a glass electrode and a reference electrode (or a combined electrode) at 20 ± 2 °C during the stirring or immediately after it; a reading is taken as stable when it changes by no more than 0.02 unit within 5 s, and it is recorded to two decimal places, while in the test report it is given to 0.1 pH. The pH meter is calibrated with at least two buffers (pH 4.00; 6.88; 9.22 at 20 °C, or commercial ones) at 20 ± 2 °C; above pH 10 an electrode for that range or an additional buffer of pH 12 is used. In samples with a high content of charged particles (organic matter, clay) the suspension effect alters the difference of potentials — it is minimised by gentle stirring; in carbonate materials the uptake of CO₂ makes the attainment of equilibrium difficult, and the drying of samples containing sulfides may lower the pH considerably.
Applications
- Routine analysis of agricultural and horticultural soils — assessment of the reaction, classification of the soil, planning of liming
- Monitoring of soils (the chemistry of arable soils, industrial sites, reclamation)
- Assessment of sewage sludges and composts (treated biowaste) before agricultural use
- Testing of forest soils, horticultural growing media and green areas
- A parameter accompanying the determination of metals, of available nutrients and of hydrolytic acidity
Key parameters
| Parameter | Value |
|---|---|
| Range | pH 2–12 |
| Sample-to-solution ratio | 1:5 by volume (≥ 5 ml of sample + 25 ml of solution; biowaste without comminution: ≥ 60 ml + 300 ml) |
| Solutions | water (conductivity ≤ 0.2 mS/m at 25 °C); KCl 1 mol/l (74.5 g/l); CaCl₂ 0.01 mol/l (1.47 g CaCl₂·2H₂O/l) |
| Time | shaking 60 ± 10 min; standing 1–3 h |
| Temperature | 20 ± 2 °C (calibration and measurement) |
| Reading and result | stability ≤ 0.02 pH within 5 s; recorded to 0.01; result to 0.1 pH |
| Precision (ISO/TC 190, 35 laboratories) | r 0.06–0.14; R 0.25–0.79 (H₂O the worst, CaCl₂ the best) |
| Presence in accreditation scopes (measured 22.09.2026) | the „Laboratories” tab shows 147 entries at 76 laboratories — that is the result of matching by the BEGINNING of the string; the exact value „PN-EN ISO 10390:2022-09” gives 140 entries at 73 laboratories. The figure 35 above refers to the ISO interlaboratory trial, not to citations in scopes — these are two different quantities |
Standard
- Standard number
- PN-EN ISO 10390:2022-09
- Title (PL)
- Gleba, uzdatnione bioodpady oraz osady ściekowe — Oznaczanie pH
- Title (EN)
- Soil, treated biowaste and sludge — Determination of pH
Step-by-step procedure
-
Laboratory sample
Use the fraction < 2 mm of an air-dry sample or of one dried at a temperature not higher than 40 °C (e.g. in accordance with ISO 11464 or EN 16179). Treated biowaste with particles of 10–40 mm is measured without comminution; field-moist samples — after homogenisation; liquid sludges — directly.
-
Suspension
Measure out with a spoon a representative portion of at least 5 ml into a bottle and add five times that volume of water, of KCl 1 mol/l or of CaCl₂ 0.01 mol/l according to the purpose. Shake it for 60 ± 10 min, then leave it to stand for 1–3 h, with access of air avoided (other conditions are permissible once the comparability of the results has been demonstrated).
-
Calibration
Set up the pH meter in accordance with the manufacturer's instructions and calibrate it with at least two buffers at 20 ± 2 °C; with electrodes in good order equilibrium is usually attained within 30 s.
-
Measurement
Measure the pH of the suspension at 20 ± 2 °C during stirring (intensive enough for the suspension to be homogeneous, without drawing in air) or immediately after it. Take the reading once it has stabilised (a change of ≤ 0.02 pH within 5 s; usually up to 1 min, longer at a high pH, in water and where there are large differences of pH between the samples) and record it to two decimal places. Note any failure to stabilise in the test report.
-
Test report
Give the reference to the standard, the identification of the sample, the solution used (or the direct measurement in a liquid sludge), the result to 0.1 pH, any difficulties with equilibrium and the deviations from the standard.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| pH meter | with slope adjustment and temperature compensation, reading to 0.01 pH | — |
| Glass electrode and reference electrode, or a combined electrode | for pH > 10 an electrode for high pH; in soils there is an increased risk of damage and of contamination of the electrodes | — |
| Shaker or stirrer | shaking of the suspension for 60 min | — |
| Thermometer or temperature probe | accuracy 1 °C | — |
| Sample bottles of ≥ 50 ml of borosilicate glass or of polyethylene with a screw cap | allowing shaking and measurement | — |
| Spoon of known capacity ≥ 5.0 ml | measuring out of the test portion by volume | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Water of a conductivity of ≤ 0.2 mS/m (25 °C) | — | the pH-H₂O suspension and the preparation of the solutions |
| Potassium chloride, c(KCl) = 1 mol/l (74.5 g in 1000 ml) | — | the pH-KCl suspension |
| Calcium chloride, c(CaCl₂) = 0.01 mol/l (1.47 g CaCl₂·2H₂O in 1000 ml) | — | the pH-CaCl₂ suspension |
| Buffer of pH 4.00 (20 °C): 10.21 g of potassium hydrogen phthalate/l (dried for 2 h at 115 °C) | — | calibration of the pH meter |
| Buffer of pH 6.88 (20 °C): 3.39 g KH₂PO₄ + 3.53 g Na₂HPO₄/l | — | calibration of the pH meter |
| Buffer of pH 9.22 (20 °C): 3.80 g Na₂B₄O₇·10H₂O/l | — | calibration of the pH meter (the buffers are stable for 1 month in PE) |
Health and safety (OHS)
- The solutions and the buffers are of little danger; protect the glass electrode against damage and against contamination with soil material
- Treat samples of contaminated soils as potentially dangerous material — gloves, avoidance of dust
- Dry the potassium hydrogen phthalate and the potassium dihydrogen phosphate before use for 2 h at 115 ± 5 °C; avoid dust when weighing out the reagents
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 10390:2022-09 — “Soil, treated biowaste and sludge — Determination of pH”.
How does this method work?
A test portion (at least 5 ml, measured out with a spoon) is suspended in five times its volume of water, of a solution of KCl (c = 1 mol/l) or of CaCl₂ (c = 0.01 mol/l), shaken for 60 ± 10 min and left to stand for at least 1 h, but for no longer than 3 h, with access of air avoided.
What is the measuring range and accuracy?
Range: pH 2–12; Sample-to-solution ratio: 1:5 by volume (≥ 5 ml of sample + 25 ml of solution; biowaste without comminution: ≥ 60 ml + 300 ml); Solutions: water (conductivity ≤ 0.2 mS/m at 25 °C); KCl 1 mol/l (74.5 g/l); CaCl₂ 0.01 mol/l (1.47 g CaCl₂·2H₂O/l); Time: shaking 60 ± 10 min; standing 1–3 h.
How long does the test take?
The procedure comprises 5 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
pH meter, Glass electrode and reference electrode, or a combined electrode, Shaker or stirrer, Thermometer or temperature probe, Sample bottles of ≥ 50 ml of borosilicate glass or of polyethylene with a screw cap, Spoon of known capacity ≥ 5.0 ml.
Where is this test used?
Routine analysis of agricultural and horticultural soils — assessment of the reaction, classification of the soil, planning of liming; Monitoring of soils (the chemistry of arable soils, industrial sites, reclamation); Assessment of sewage sludges and composts (treated biowaste) before agricultural use; Testing of forest soils, horticultural growing media and green areas; A parameter accompanying the determination of metals, of available nutrients and of hydrolytic acidity.
What safety precautions apply?
The solutions and the buffers are of little danger; protect the glass electrode against damage and against contamination with soil material; Treat samples of contaminated soils as potentially dangerous material — gloves, avoidance of dust; Dry the potassium hydrogen phthalate and the potassium dihydrogen phosphate before use for 2 h at 115 ± 5 °C; avoid dust when weighing out the reagents.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 10390.